You'll see the difference in the sense than a Z270 will not work with a 9900K at all and a Z390 will?
An i9 can certainly pair well with much more powerful GPUs than a 980TI - but a 980TI is no slouch, even 4 1/2 years after launch. You're looking at roughly 1070 levels of performance, so I'd say you can hold off upgrading the GPU a while yet.